1. Field of the Invention
This invention generally relates to a method of brazing stainless steel components and, more particularly, to brazing complex shapes of stainless steel. The invention relates to a method of brazing, accompanied by a subsequent controlled cooling and quenching treatment for forming an integral assembly from a plurality of separate stainless steel components of complex geometry to prevent cracking thereof. In addition, the invention relates to an article made in accordance with the method, most preferably, a stainless steel impeller assembly.
2. Background Art
Welded assemblies, such as impellers, are conventionally formed initially from two or more separate parts. For an impeller assembly, one part is called the hub, which may or may not contain integrally machined or welded blades. The other is called the cover, which may or may not contain integrally machined or welded blades. These two parts, with at least one part containing integrally machined or welded blades, have conventionally been joined by a brazing method to form an integral assembly called an impeller. Due to the required mechanical properties of the base materials used, a subsequent cooling and quenching treatment is required.
Traditional brazing methods initiate a quenching treatment at a temperature that is just slightly below the liquid phase of the brazing alloy to ensure that the mechanical properties of the base materials are satisfied. While this traditional method is beneficial to the base materials, there is evidence that the strength of the brazed joint is inadequate at these quenching temperatures and, thus, occasionally results in cracking of the brazed joint. This is particularly true in complex shapes, such as an impeller assembly, having cross-sectional thickness variations which cause thermally induced stress upon cooling.
The problem of brazed joint strength has been addressed in a number of ways. U.S. Pat. No. 5,190,596 to Timsit discloses a method of brazing metal surfaces, which utilizes a mixture of a metal and a brazing flux. The metal of the mixture is adapted to form a brazable eutectic with one or both of the surface metals. U.S. Pat. No. 6,112,971 to Castaldo et al. discloses brazing 304L stainless steel parts using an Ni-13Cr-2.8B-4Si-4Fe brazing alloy. U.S. Pat. No. 6,074,604 to Saito et al. discloses a brazing filler metal for brazing stainless steel at low temperatures so as not to adversely affect the properties of the stainless steel while minimizing brittleness in the brazed joint. The brazing filler metal includes 5% to 30% by weight of Mn or Sn, 20% to 70% by weight of Cu, and inevitable impurities, with the balance being Ni. Each of the cited references, while providing some improvement in brazed joint strength and longevity, do not provide the level of brazed joint strength required for high stress applications, such as, for example, the brazed joints of an impeller. Particular concerns in this regard include the development of cracks of the brazed joint and thermal distortion, which is a problem in brazed impeller assemblies.
There is an established need for a brazing method that can provide impeller assemblies with brazed joints having adequate mechanical properties and that are not prone to cracking of the brazed joint.